It will be appreciated by those skilled in the art that in order to use an integrated circuit chip, it is necessary to make electrical connection to the integrated circuit incorporated within the chip. Traditionally, these connections have been made through contact pads located at one main face (hereinafter called the interconnect face) of the chip, utilizing bond wires. A chip is mounted on a substrate, e.g. of ceramic material, with its interconnect face uppermost, and a wire bonder is used to "stitch" bond wires between the IC pads and corresponding pads on the substrate. In a wire bonder, the bond wire is held against the pad during bonding by a rod-like or tubular tool. The dimensions of the tool, dictated by the need for rigidity, place a fairly large lower limit on the spacing between the pads on the IC. Typically, the pads are on at least six mil (one mil equals 0.001 inch, or approximately 0.025 mm.) centers. As the complexity of integrated circuits has increased, giving rise to demands for larger numbers of data and power supply lines, so has the scale of integration increased, reducing the needed size of the chip. The point has been reached where integrated circuit chips are being made larger than would be dictated by the complexity of the circuits in order to provide a sufficiently large interconnect face to accommodate the desired number of contact pads. In addition, the bond wire itself may introduce an undesirably large uncompensated inductance in the connection path.
Another method of mounting a chip to a substrate is known as the "flip-chip" technique. By this method, the substrate, e.g. of ceramic material, carries conductive strips that terminate at the perimeter of the footprint of the chip in a pattern of termination points corresponding to the pattern of pads on the chip, and the chip is placed on the substrate with its interconnect face down and its bond pads in direct physical contact with the termination points. The bond pads or the termination points carry solder preforms, and by applying heat and pressure through the chip a direct metallurgical bond is established between the chip's contact pads and the substrate's termination points. Through use of a multilayer ceramic substrate the flip-chip technique allows the uncompensated inductance in the connections to the pads to be reduced. However, thermal mismatch between the chip and the substrate affects the reliability of the connections and may lead to damage to the chip.
A further problem that arises with conventional IC packaging techniques is that of heat dissipation. In the conventional ceramic packaging for IC chips, in which the chip is mounted on a ceramic substrate and the ceramic substrate is secured to an etched circuit board, the thermal flux through the substrate limits the packing density for discrete IC packages on the circuit board.